Heat-treatment of zinc/aluminium alloys

ABSTRACT

Zinc/aluminium alloys containing 18 to 40 percent aluminium are rendered superplastic by annealing in the single-phase solid region, cooling to below 275* C at not more than 10* C per minute, and, simultaneously with and/or subsequently to cooling, working to at least 90 percent reduction above 200* C at least half of which working is carried out below 275* C. Optionally the alloy is cooled below 200* C and then reheated to the given range for some or all of the working.

United States Patent 1191 1111 3,753,791 Swanson Aug. 21, 1973 [54] HEAT-TREATMENT OF ZINC/ALUMINIUM 2,169,441 8/1939 Winter et a1. 148/116 R ALLOYS 2,982,677 /1961 Pelzel 75/178 AC 3,420,717 l/l969 Fields, Jr. et al. l48/ll.5 R [75] inventor: Colin John Swanson, l-lorfield,

England FOREIGN PATENTS 0R APPLICATIONS [73] Assignee: Imperial Smelting Corporation 2%; gg g figzj ls/I78 (Alloys) Limited, London, England i Filed; J 1971 Primary Examiner-W. W. Stallard [21] APPL No 103,911 Attorney-Lane, Aitken, Dunner 81: Ziems Foreign Application Priority Data [57] ABSTRACT Jan. 1, 1970 Great Britain 29/70 Zinc/a1luminium alloys containing 13 to Percent minium are rendered superplastic by annealing in the 52 US. Cl. l48/l1.5 R, /178 AM -Ph Solid o cooling to below C at [51] Int. Cl. C221 17/00 not more than C 1 minute, and, Simultaneously [58] Field of Search 148/1 1.5 R; with and/or Subsequently to cooling, working to at least 75 17g A 7 AM, 173 AT) 73 AN percent reduction above 200 C at least half of which working is carried out below 275 C. Optionally 5 References Cited the alloy is cooled below 200 C and then reheated to UNITED STATES PATENTS the given range for some or all of the working.

1,945,288 1/1934 Morell 75/178 10 Claims, No Drawings HEAT-TREATMENT OF ZINC/ALUMINIUM ALLOYS This invention relates to the heat treatment of zinc- /aluminium alloys to provide them with superplastic properties.

The term superplastic is used herein to refer to metals which have a substantial strain rate sensitivity. Strain rate sensitivity m is the exponential variable in the expression a= KE", vvherein '11 represents stress in load per unit area, represents strain rate in terms of length change per unit time and K represents a constant which is termed the strain rate coefficient.

While the existence of a substantial strain rate sensitivity in determines the ultimate elongation to which a metal may be subjected without fracture, the strain rate coefficient K is significant in determining the strength of the material and hence the amount of work required to form the part.

Processes for rendering a zinc/aluminium alloy superplastic comprise heat treating the alloy at a temperature above 275 C for a period of time to allow the alloy to become substantially homogenous, quenching or similarly rapidly cooling the alloy and mechanically working the cooled alloy at a temperature below 275 C.

We have now discovered that, when working with zinc/aluminium alloys containing 18 to 40 percent by weight of aluminium it is possible to modify this heat treatment/cooling/working process by employing 'a relatively slow rate of cooling, as opposed to quenching, provided that certain precautions are taken during the working process.

The present invention consists in a process for providing superplastic properties in alloys of zinc and aluminium, comprising a. heat-treating a body of the alloy containing between 18' and 40 percent by weight of aluminium, the remainder being zinc together with any incidental impurities and minor ternary alloying components and the alloy composition and temperature being such as to fall within the aluminium rich single-phase region of the zinc-aluminium phase diagram, until a substantially ho mogeneous structure is obtained, and thereafter characterized by:

b..cooling the body of alloy to a temperature below 275 C at a cooling rate not in excess of C per minute, and r c. working the body of, alloy to reduce a dimension thereof by at least 90 percent, at least half of the total percentage dimension reduction being carried out below 275 C and all the said percentage dimension reduction being'carried out above 200 C.

The reduction is more preferably at least 92 percent, or at least 94 percent; that is to say, the dimensions of the work piece are less than 10 percent, and preferably less than 6 percent, those of the original body of alloy. Working is preferably effected by rolling.

The cooling rate of less than 10 C per minute is greatly different from a quenching rate, which is usually several degrees per second. It can be achieved by cooling in air (normally in still air) or by cooling by discontinuing the supply of heat to the heat-treating zone, such as a furnace, and allowing the whole environment of this nature to cool. Furnace cooling is generally slower than air cooling; preferably a cooling rate of 3 to 5 C per minute is utilized.

Other preferred features include a heat treatment time of at least 3 hours (less than this is usually though not invariably inadequate for full homogenization) and an optional additional working stage below 200 C, especially for the lower aluminium alloys within the range.

A key feature of this invention is the combination of the manipulative steps of cooling and working, carried out with reference to the temperature range of 200 to 275 C and preferably 200 to 250 C. Thus, provided the above-recited conditions are fulfilled, some of the working may be effected above 275 C but most in the range 200 275 C, or all of the working can be effected in this range. Since it is possible to operate so that cooling is effected to below 200 C, the body of alloy is reheated to above 200 C and subsequently at least part of said total reduction is carried out at below 275 C, other possibilities present themselves. Thus, all of the (at least 90 percent) reduction can be effected after such reheating. Altematively it is conceivable to work the body partly before cooling below 200 C (e.g. to room temperature) such working being wholly or only partly below 275 C,.and then to workthe alloy after reheating; in all such instances the work in the 200 275C range should be half the total which total itself should be at least 90 percent of original dimensions.

Another aspect of the invention is the superplastic alloy produced by the above process. To be generally useful, this should have a vacuum forming time, measured for a sheet 0.05 inches thick by the method described below, of below 350 seconds; however, this value is not critical, and depends upon the intended end use of the material. Thickness of sample affects this VFT, although not in a linear fashion.

Such an alloy can be of zinc and aluminium with only incidental impurities. The eutectoid composition can be used, but the so-called /30 zinc/aluminium alloy, within the compositional tolerances common in this art, has also been found valuable. Ternary alloying components can be used; thus magnesium can be present,:e.g., in amounts up to 0.1 percent by weight (specifically 0.01 percent) in amounts up to 1.0 percent by weight, specif cally0.5 percent. I

The invention will be further explained and described with reference to the following non-limiting examples. In each of these examples reference is made to Vacuum forming time or VFI'. This is a known measure of superplasticity and is carried out by (i) clamping a disc of alloy over the end of a tube of internal diameter 3.2 inches maintained in the thermostatted air enclosure, (ii) applying vacuum to one side of the disc and (iii) measuring the time taken to form the disc into a part of a hemisphere of 1.15 inches radius, i.e., to increase the relevant area by 50 percent. A suitable probe is used to establish when the hemispherical condition is reached.

Throughout these examples annealing" and homogenization are used synonymously.

EXAMPLE 1 A body of alloy 0.75 inches thick containing by weight 70 percent zinc, 30 percent aluminium and 0.5 percent copper (measured on the basis of zinc and aluminium present) was annealed at 375 C for 5 hours, air-cooled to room temperature, reheated to 250 C and rolled to 0.05 inches thick at this temperature. The vacuum forming time was 255 seconds.

A like body of alloy annealed at 375 for 1 hour, quenched, aged for 1 hour at room temperature and rolled to 0.05 inches at 250 C gave a similar vacuum forming time of 245 seconds. Thus the two procedures appear to give much the same result.

EXAMPLES 2 TO 8 Alloy stock 0.75 inches thick and of a weight composition 70 percent zinc, 30 aluminium and 0.01 percent magnesium, measured on the basis of total zinc and aluminium, was heat-treated, variously quenched or aircooled to room temperature, and thereafter reheated and rolled at 250 C to 0.05 inches. Details of the procedure were as follows:

Homogenized for Cooled by VFT in seconds Ex. 2: l hour at 350C Quenching I40 Ex. 3: 1 hour at 375C Quenching 245 Ex. 4: l hour at 375C Air-cooling 1110 Ex. 5: 5 hours at 375C Air-cooling 265 Ex. 6: 1 hour at 400C Quenching I80 Ex. 7: l hour at 400C Air-cooling 580 Ex. 8: 5 hours at 400C Air-cooling 180 Examples 2, 3 and 6 illustrate a prior art procedure, while Examples 4 and 7 illustrate the effects of insufficient homogenization time. Examples 5 and 8 illustrate the present invention. Air-cooling takes place at about 5 C per minute.

EXAMPLE 9 This illustrates that the initial type of alloy body has some effect on results, but that the invention is of general applicability.

Rolling slabs of 0.75 inches thickness produced by (A) gravity casting and (B) direct chill semicontinuous casting, and of 70/30 zinc/aluminium composition were annealed at 375 C and then air-cooled to room temperature. They were then reheated to, and rolled at, 250 C to 0.05 inches. Results were as follows:

Annealing Time in Hours VF! in Seconds (A) (B) 1 H 270 465 16 245 2| 275 40 310 It will be apparent that required annealing times to achieve an acceptable VFT of, e.g., 350 or below are different, but that with adequate annealing time no essential differences are apparent.

EXAMPLE 10 This illustrates that while adequate working must be effected below 275 C, it is possible to carry out some working above that temperature without diminishing eventual superplasticity.

An 0.750-inches thick gravity-cast rolling slab of 70/30 zinc/aluminium composition was annealed for 5 hourst and 375 C. Thereafter it was subjected to rolling procedures as follows, with effects as shown:

Treatment VFT in seconds i. No cooling; rolled from 0.75 to 0.05 inches at 375C 2300 ii. furnace cooling; rolled from 0.75 to 0.05 inches at iii. furnace cooling; rolled from 0.75 to 0.05 inches at 325C 660 iv. furnace cooling; rolled from 0.75 to 0.05 inches at 300C 960 v. furnace cooling; rolled from 0.75 to 0.05 inches at EXAMPLE ll This indicates that too much rolling at above 275 C can be detrimental to superplasticity.

Gravity-cast slabs of /30 zinc/aluminium alloy were annealed then 5 hours at 375 C. Thereafter they were subjected to different procedures.

One slab was air-cooled and aged for 24 hours at room temperature and then reheated to and rolled at 250 C to 0.05 inches. It had a VFT of 285 seconds.

The other slab was not cooled, but rolled at 375 C to 0.375 inches and then air-cooled and aged for 24 hours at room temperature before being reheated to, and rolled at 250 C to 0.05 inches. It had a VFT of 720 seconds.

The difference in the results can be partly attributed to insufficient working below 275 C; it seems possible moreover, by comparison with Example 10, that the reheating procedure, which is fairly lengthy since the stock is 0.375 inches thick, in some way makes matters worse.

EXAMPLE 12 This indicates that the requisite amount of working must take place above a minimum temperature ofabout 200 C as well as below 275 C.

Three samples of gravity-cast 0.75 inches thick rolling slab of a basic 70/30 zinc/aluminium composition,

but containing 0.5 percent of copper by weight of the total-zinc and aluminium were annealed for 5 hours at 375 C and air-cooled to room temperature. They were thereafter treated as follows:

A. Reheated to 250 C and rolled to 0.05 inches to give, according to the invention,an acceptable VFT of 255 seconds,

B. Reheated to C (Le. below the temperature range of the invention) and rolled to 0.05 inches to give a VFT of 405 seconds which is outside of the usually acceptable limits,

C. Reheated to 250 C and rolled to 0.25 inches; then cooled to room temperature, and reheated to 150 C and rolled to 0.05 inches to give a VFT of 670 seconds. The percentage reduction (from 0.75 to 0.25 inches) is below that of the present invention and subsequent working below the temperature range of the invention cannot improve the situation. It is again to be surmised that reheating, in this case twice, is likely to cause detrimental grain growth.

EXAMPLE 13 This indicates that if sufficient reduction has taken place above 200 C, and that if a suitable proportion has taken place below 275, further working does not destroy superplasticity.

Samples of rolling slab of the same composition as in Example 12 but 5 inches thick were annealed for 24 hours at 360 C, furnace cooled to 250 C, rolled at 250 to 0.25 inches thickness and allowed to cool to room temperature. These samples were superplastic, and subsequent working as follows did not destroy superplasticity. a. Reheat to 250 C and roll to 0.05 inches VFT 200 b. Reheat to 150 C and roll to 0.05 inches VFT 190 c. Reheat to 250 C and roll to 0.025 inches VFT 165 d. Reheat to 100 C and roll to 0.025 inches VFT EXAMPLE 14 This example illustrates how reheating time can affect VFT, in some instances more than actual working temperatures used.

An homogenized ingot of 70/30 zinc/aluminium, 3.75 inches thick was rolled to 0.08 inches at 250 260 C, the sheet thus obtained having a VFT of 260 seconds. A number of samples were rolled to 0.025 inches at the following temperatures to give VFT values at follows:

TC VFT (in seconds) 250 55 150 75 Room Temp. 55

Another sample was held at 250 C for 3 hours prior to rolling, at 250 C, to 0.025 inches thickness. This sample gave a VFT of 130 seconds.

I claim:

1. A process for providing superplastic properties in alloys of zinc and aluminium, comprising:

a. heat-treating a body of the alloy containing between l8 and 40 percent by weight of aluminium, the remainder being zinc together with any incidental impurities and minor ternary alloying components and alloy composition and temperature being such as to fall within the aluminum-rich single-phase region of the zinc-aluminium phase diagram, until a substantially homogeneous structure is obtained, and thereafter characterised by:

b. cooling the body of alloy to a temperature below 275 C at a cooling rate not in excess of 10 C per minute, and

0. working the body of alloy to reduce a dimension thereof by at least percent, at least half of the total percentage dimension reduction being carried out below 275 C and all the said percentage dimension reduction being carried out above 200 C.

2. A process as claimed in claim 1, wherein some of said total reduction is carried out at above 275 C.

3. A process as claimed in claim 1, wherein all of said total reduction is carried out at below 275 C.

4. A process as claimed in claim 3, wherein cooling is effected to below 200 C, the: body of alloy is reheated to above 200 C and below 275 C and subsequently at least part of said total reduction is carried out at below 275 C.

5. A process as claimed in claim 4, wherein all of said total reduction is effected after said reheat.

6. A process as claimed in claim 1, wherein at least half of said total reduction is carried out below 250 C.

7. A process as claimed in claim 1, wherein the heattreatment period is at least 3 hours.

8. A process as claimed in claim 1, wherein the cooling rate is from 3 to 5 C per minute.

9. A process as claimed in claim 1, wherein working is effected by rolling.

10. A process as claimed in claim 1, wherein a subseprovided below 200 C.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,753, 791 2 Dated Auqust 2 1, 1973 Inventor(s) Colin John Swanson It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line ll, after "area," add E Column 4, line 27, after "annealed cancel "then" and substitute fpr Column 5, line 31, after "values" cancel "at" and substitute Signed and sealed this 26th dayof February 197M.

(SEAL) Attest:

' ANN EDWARD M.FLETCHER JR. A H D Attesting Officer I COIIIJIllSSlOIlQT of Patents 

2. A process as claimed in claim 1, wherein some of said total reduction is carried out at above 275* C.
 3. A process as claimed in claim 1, wherein all of said total reduction is carried out at below 275* C.
 4. A process as claimed in claim 3, wherein cooling is effected to below 200* C, the body of alloy is reheated to above 200* C and below 275* C and subsequently at least part of said total reduction is carried out at below 275* C.
 5. A process as claimed in claim 4, wherein all of said total reduction is effected after said reheat.
 6. A process as claimed in claim 1, wherein at least half of said total reduction is carried out below 250* C.
 7. A process as claimed in claim 1, wherein the heat-treatment period is at least 3 hours.
 8. A process as claimed in claim 1, wherein the cooling rate is from 3* to 5* C per minute.
 9. A process as claimed in claim 1, wherein working is effected by rolling.
 10. A process as claimed in claim 1, wherein a subsequent working step is provided below 200* C. 